Escherichia coli 70S ribosomes are complex macromolecular machines consisting of 3 ribosomal RNA (rRNA) molecules and 54 ribosomal proteins (r-proteins). 70S ribosomes are capable of sequence-defined polymerization of 20 amino acid monomers into proteins with a wide variety of biological functions. In vitro ribosome studies have elucidated ribosome structure, r-protein assembly, and translational mechanisms.
In vitro assembly, or reconstitution, of Escherichia coli ribosomes from purified native ribosomal components into functionally active small (30S) and large (50S) ribosomal subunits was first achieved in pioneering works ˜40 years ago (Nierhaus K H & Dohme F, “Total reconstitution of functionally active 50S ribosomal subunits from Escherichia coli.” Proc. Natl. Acad. Sci., U.S.A. 71, 4713-4717 (1974); Traub P & Nomura M, “Structure and function of E. coli ribosomes. V. Reconstitution of functionally active 30S ribosomal particles from RNA and proteins,” Proc. Natl. Acad. Sci., U.S.A. 59, 777-784 (1968)). The conventional 30S subunit reconstitution protocol involves a one-step incubation at 20 mM Mg2+ and 40° C. (see, for example, (Traub & Nomura (1968); Church, G M & Jewett, M C, U.S. Patent Application Publication US20120171720A1, published Jul. 5, 2012 and entitled “Method of Making Ribosomes”), and can be facilitated at lower temperatures by chaperones (Maki J A & Culver G M, “Recent developments in factor-facilitated ribosome assembly.” Methods 36, 313-320 (2005)). The conventional 50S subunit reconstitution protocol involves a non-physiological two-step high-temperature incubation, first at 4 mM Mg2+ and 44° C., then at 20 mM Mg2+ and 50° C. (Nierhaus & Dohme (1974); Church & Jewett (2012)).
Studies using the conventional reconstitution approach have revealed many important insights into ribosome assembly (Nierhaus K H, Reconstitution of ribosomes, in Ribosomes and Protein Synthesis, A Practical Approach, Oxford: Oxford University Press, (1990). Yet inefficiencies in reconstitution make the construction and analysis of engineered variants difficult (Semrad K & Green R, “Osmolytes stimulate the reconstitution of functional 50S ribosomes from in vitro transcripts of Escherichia coli 23S rRNA,” RNA, 8, 401-411 (2002)). For example, conventionally reconstituted 50S subunits made with in vitro-transcribed 23S rRNA (lacking the naturally occurring post-transcriptional modifications) are up to 10,000 times less efficient in reconstitution than those using mature 23S rRNA as measured by the fragment reaction, where single peptide bonds are formed on isolated 50S subunits (Semrad & Green (2002)). Furthermore, the non-physiological two-step conditions for 50S assembly preclude coupling of ribosome synthesis and assembly in a single, integrated system.
Ribosome biogenesis is still not fully defined, as some RNases involved in rRNA processing are unidentified, while in vitro ribosome reconstitution studies using purified rRNA may not accurately reflect the simultaneous in vivo processes of rRNA synthesis and ribosome assembly (Wilson D N & Nierhaus K H, “The weird and wonderful world of bacterial ribosome regulation,” Critical reviews in biochemistry and molecular biology 42, 187-219 (2007)). In addition, attempts at engineering the ribosome to introduce new functionalities are severely limited by cell viability constraints. Orthogonal ribosomes provide one route, but they must be separated from native ribosomes required for cell growth and may still be toxic to cells (Barrett, O P & Chin, J W, “Evolved orthogonal ribosome purification for in vitro characterization,” Nucleic Acids Res, 38, 2682-2691 (2010); Cochella L & Green R, “Isolation of antibiotic resistance mutations in the rRNA by using an in vitro selection system,” Proc. Natl. Acad. Sci., U.S.A. 101, 3786-3791 (2004)). Meanwhile, attempts to assemble ribosomes from in vitro transcribed and purified rRNA using classical reconstitution methods has proven unsuccessful, likely due to the need for post-transcriptional modification of the 23S rRNA (Traub & Nomura (1968); Nierhaus & Dohme (1974); Green R & Noller H F “In vitro complementation analysis localizes 23S rRNA posttranscriptional modifications that are required for Escherichia coli 50S ribosomal subunit assembly and function,” RNA, 2, 1011-1021 (1996)); Semrad & Green (2002)). The direct study of ribosome biogenesis in vitro necessitates removal of the complication of cell viability.
The integrated synthesis, assembly, and translation (iSAT) technology was developed for in vitro 70S ribosome biogenesis to circumvent several of the limitations to previous in vitro translation systems using reconstituted ribosomes (Church and Jewett, (2012); Jewett M C et al., “In vitro integration of ribosomal RNA synthesis, ribosome assembly, and translation,” Mol Syst Biol. 9:678 (2013)). This technology allows for synthesis of rRNA from individual plasmids, assembly with purified total protein of 70S ribosomes (TP70), and translation of a reporter protein such as luciferase or superfolder GFP (sfGFP) as a measure of ribosome activity (FIG. 1) (Jewett et al. (2013)). These processes all occur simultaneously in vitro at 37° C. A near-physiological salt conditions of this technology allow these biological processes to be active at 37° C. without magnesium shifts previously required for ribosome constitution from purified components (see, for example, Jewett et al. (2013)).
However, iSAT technology as previously reported showed limitations in efficiency leading to low ribosomal activity (Jewett et al. (2013)). Full 70S iSAT ribosomes showed 8-fold lower activity than ribosomes assembled in the same system from purified total rRNA of 70S ribosomes (TR70) and TP70, suggesting a discrepancy between in vitro synthesized rRNA and purified native rRNA. Previous iSAT methods focused on individual subunit assembly to improve reporter signal in translation assays. Yet present iSAT systems maintain bottlenecks that limit the iSAT process and bar increased ribosome activity.